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Revision as of 03:06, 30 July 2013 by Weegee121 (talk | contribs) (Natural Satellites)
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Jool is a gas giant and the sixth planet of the Kerbol star system. It is the Jupiter analog for Kerbal Space Program. Aside from Kerbol, Jool has the largest diameter and greatest mass of all celestial bodies. While its distance from Kerbin makes it difficult to reach, it is one of the most appealing targets for missions due to its large and complex system of five moons: Laythe, Vall, Tylo, Bop, and Pol.


Aerobraking in Jool's atmosphere.

Although Jool has a radius ten times greater than Kerbin, its atmosphere begins just twice as high (138.2 km). It is extremely dense, well suited for aerobraking from a high-speed interplanetary intercept. Orbiting just outside the atmosphere will give you a period of 94.7437 minutes, and a speed of 6,785 m/s. You typically do not need to go lower than 100 km to aerobrake.

In general, the atmospheric pressure on Jool at an altitude expressed in meters is:

The effects of landing on Jool's "surface" as a Kerbonaut

Closer to the surface, the physics of the game start to break down. Due to its extremely thick atmosphere, terminal velocity is so low that lighter spacecraft will not even need parachutes or retrorockets. It is not impossible to land on its solid surface, though spacecraft are inevitably and invariably devoured by the Kraken. If the craft is very small, it may survive the Kraken, but the parts will explode once they are 100m below the surface. The message "... collided with Cloud" will display in the mission summary.

If a Kerbonaut is put on EVA he will not be destroyed, making one-way sacrificial landings possible. However, as seen near the end of this video, the Kerbonaut will start to glitch out and shake uncontrollably. This only happens if you're on time warp 4. The game will later glitch and cut to a black screen with the altimeter going through numbers rapidly. Unless you restart the game, Kerbin's atmosphere will be missing.

This glitch, while most common on Jool, is not unique to Jool, and can even be experienced on Kerbin in extreme situations; for example, going on EVA from a high speed SpacePlane as it takes off can occasionally cause this, or it can even (though much rarer) happen in less intense situations as simple as sliding slowly on the Mun. If you do experience this outside of Jool, it is very difficult to replicate, which is why recordings of this are so rare.

Reference Frames

Time warp Minimum Altitude
5× 200 000 m (above the atmosphere)
10× 200 000 m (above the atmosphere)
50× 200 000 m (above the atmosphere)
100× 200 000 m (above the atmosphere)
1 000× 300 000 m
10 000× 600 000 m
100 000× 1 200 000 m

Natural Satellites

The ocean moon Laythe, about to transit Jool.

Jool has five natural satellites, each with an orbit well-aligned with Jool's orbital plane:

  • Laythe, an ocean moon, is the only moon with an atmosphere. It is the closest to Jool and second largest of its moons. Due to its high orbital velocity, it is somewhat challenging to reach. Next to Eve, its size and composition make it the most similar celestial body to Kerbin.
  • Vall, an ice moon, is the third largest and second closest of Jool's moons. Its orbital path and velocity sit almost exactly between Laythe and Tylo.
  • Tylo, a rocky moon, has gravity similar to Kerbin and terrain similar to Kerbin's Mun. It has the largest SOI of Jool's moons, making it easy to encounter, but achieving orbit and landing are exceptionally difficult due to its large gravity well and lack of atmosphere.
  • Bop, a captured asteroid, is the fourth largest of Jool's moons. Due to its distant, erratic orbit and low gravity, it is also challenging to reach.
  • Pol, named after a Pollen grain which it resembles, is Jool's smallest and most distant moon. It is yellow and green, and its terrain is rocky and uneven, with tall, spiky mountains.

Laythe, Vall, and Tylo are in a Laplace resonance, with orbital periods of 1:2:4 respectively. Despite the fact that the moons can easily eclipse both each other and Jool, they do not.

Synchronous Orbits around any of the Joolian moons are impossible, as they all lie outside the Sphere of Influence of the moons, as is common with tidally locked bodies.

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  • Jool has roughly the same equatorial radius as the planets Earth and Venus.
  • The sunsets and sunrises on Jool are a shade of yellowish brown.
  • Like Jool, Jupiter's nearest three moons (Io, Europa, and Ganymede) have a Laplace resonance of 1:2:4.
  • Jool, Laythe and Tylo all have an surface gravity of 7.85 m/s² (≃ 0.8 g)


  • Added a new, more distant moon named Pol.
  • Initial Release